Add IsFree subscription toggle to bypass all tier limitations

- Add IsFree boolean field to UserSubscription model
- When IsFree is true, user sees limitations_enabled=false regardless of global setting
- CheckLimit() bypasses all limit checks for IsFree users
- Add admin endpoint GET /api/admin/subscriptions/user/:user_id
- Add IsFree toggle to admin user detail page under Subscription card
- Add database migration 004_subscription_is_free
- Add integration tests for IsFree functionality
- Add task kanban categorization documentation

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

internal/testutil/testutil.go

@@ -48,9 +48,11 @@ func SetupTestDB(t *testing.T) *gorm.DB {
 		&models.APNSDevice{},
 		&models.GCMDevice{},
 		&models.UserSubscription{},
+		&models.SubscriptionSettings{},
 		&models.TierLimits{},
 		&models.FeatureBenefit{},
 		&models.UpgradeTrigger{},
+		&models.Promotion{},
 	)
 	require.NoError(t, err)